14 THE CARDIOVASCULAR SYSTEM: BLOOD VESSELS, BLOOD FLOW, AND BLOOD PRESSURE
Physical Laws Governing Blood Flow and Blood Pressure
The flow rate of a liquid (the volume flowing per unit or time) through a pipe is directly proportional to the
difference between the pressures at the two ends of the pipe (the pressure gradient) and inversely proportional
to the resistance of the pipe.
FLOW = pressure gradient / resistance = P/R
The quantity P, the size of the pressure gradient, represents the driving force that pushes the flow of liquid
through a pipe; the quantity R, the resistance, is a measure of the various factors that hinder the flow of liquid
through a pipe.
PRESSURE GRADIENTS IN THE CARDIOVASCULAR SYSTEM
Whenever there is a difference in pressure between two locations, the pressure gradient drives the flow from a
region of higher pressure to one of lower pressure, or down the pressure gradient.
The driving force for bulk flow is always a pressure gradient, and the direction of flow is always down the
gradient from a region of greater pressure to a region of lower pressure.
THE ROLE OF RPESSURE GRADIENTS IN DRIVING BLOOD FLOW
o The primary function of the heart is to generate the pressure that drives the flow of blood through the
o By pumping blood into the arteries, the heart raises, mean arterial pressure, which creates a difference
in pressure between the arteries and veins that drives the flow of blood.
o The vertical distance from the vessel to the surface of the liquid, the so-called hydrostatic column,
determines the pressure at either end of the vessel. Pressure also depends on density.
o A higher hydrostatic column corresponds to a greater pressure.
o If the difference between the levels remains constant, the flow does not change.
PRESSURE GRADIENTS ACROSS THE SYSTEMIC AND PULMONARY CIRCUITS
o Mean arterial pressure (MAP, the average pressure in the aorta throughout the cardiac cycle) is about
o Central venous pressure (CVP) is approximately 2-8 mm Hg, and the pressure in the vena cava just
outside the right atrium is approximately 0 mm Hg.
o The difference between the MAP and CVP is the pressure gradient that drives blood flow through the
o Because central venous pressure is so small, we ignore.
o The pressure gradient P driving blood flow through the systemic circuit is equated to the mean arterial
o Blood flow through the pulmonary circuit is also driven by a pressure gradient – the difference between
the pressure in the pulmonary arteries and the pressure in the pulmonary veins.
o Pressure gradient is smaller than the one that goes through the systemic circuit because pulmonary
arterial pressure is lower than aortic pressure.
o Pulmonary arterial pressure = 15mm Hg. Aorta = 85mm Hg. Pulmonary venous pressure = 0mm Hg.
o If the pressure gradient to drive blood flow through the pulmonary circuit is relatively low, then the
resistance must also be low.
RESISTANCE IN CARDIOVASCULAR SYSTEM
If the pressure gradient in the pulmonary circuit is lower than that in the systemic circuit, to have the same
blood flow, the pulmonary circuit offers less resistance, so a smaller pressure gradient can achieve the same
RESISTANCE OF INDIVIDUAL BLOOD VESSELS
o A vessel with higher resistance yields a lower flow. Blood flow is greater when resistance is lower
because it is easier for blood to flow.
o Resistance depends on the physical dimensions of the tube and the properties of the fluid flowing
through it – the tube’s radius and length, and the fluid’s viscosity.
1. Vessel radius: as radius decreases, resistance increases. Vasoconstriction: decrease in blood vessel
radius. Vasodilation: increase in vessel radius.
2. Vessel length: longer vessels have greater resistance, but resistance is rarely due to changes in vessel
length. 14 THE CARDIOVASCULAR SYSTEM: BLOOD VESSELS, BLOOD FLOW, AND BLOOD PRESSURE
3. Blood viscosity: vascular resistance increases as viscosity increases. Determined by the concentration of
cells and proteins in the blood.
Overview of the Vasculature
Arteries and smaller arterioles carry blood from the heart and to capillaries, which are drained by venules, and
then larger veins, which return the blood to the heart.
The arterioles, capillaries, and venues can be seen only with the aid of a microscope and, therefore, are called
Lumen: blood vessels’ hollow interior; it is lined by a layer of epithelium called endothelium.
Capillaries, consists of a layer of endothelial cells and a basement membrane.
The walls of all other blood vessels contain various amounts of smooth muscle and fibrous and/or elastic
connective tissue. Within the fibrous connective tissue are extracellular fibers made of a protein called collagen,
which lends tensile strength to vessel walls.
Elastic connective tissue contains fibers of a highly stretchable extracellular protein called elastin, which enables
blood vessels to expand or contract as the pressure of blood within them changes.
Arteries conduct blood away from the heart and toward the body’s tissue.
Aorta (largest artery) has an internal diameter of 12.5mm and a wall that is 2mm thick.
The smaller arteries that branch off the aorta have internal diameters ranging from 2mm to 6mm and a wall
thickness of about 1mm, and this branch into yet smaller-diameter arteries.
The walls of large arteries contain large amounts of elastic and fibrous tissue, enabling arteries to withstand
relatively high blood pressures, which are higher in these vessels than anywhere else in the vasculature.
As the arteries branch into smaller arteries, the amount of elastic tissue in the walls decreases while the amount
of smooth muscle increases.
Arteries less than 0.1mm are called muscular arteries (lose most of their elastic properties).
ARTERIES: A PRESSURE RESERVOIR
The thickness of arterial walls, coupled with the relative abundance of elastic tissue, gives arteries both stiffness
and the ability to expand and contract as the blood pressure rises and falls with each heartbeat.
As pressure reservoirs, it ensures a continual, smooth flow of blood through the vasculature even when the heart
is not pumping blood (diastole).
This elastic force is stored such that during diastole, when no more blood is entering the arteries, the walls
passively recoil inward, propelling blood forward.
The pulse is caused by a pressure wave that travels along the arteries in response to blood being pushed into the
arteries during systole, causing the arterial wall to expand.
Compliance: a measure of the relationship between the pressure and volume changes.
In vessels with low compliance, such as arteries, a small increase in blood volume causes a large increase in
blood pressure (or a large increase in pressure causes only a small degree of expansion of the blood vessel
When the heart ejects blood into the arteries during systole, and causes them to expand, the resulting rise in
pressure is greater than it would be it arteries’ compliances were higher.
The low compliance of arteries is a function of elasticity of the vessel walls.
ARTERIAL BLOOD PRESSURE
Arterial blood pressure: pressure in the aorta; pressure in the aorta doesn’t stay elevated because during
diastole, blood quits flowing into the aorta yet continues to flow out, which causes a slow decline in arterial
blood pressure to a minimum just prior to the next systole.
The maximum pressure that occurs during systole is called the systolic pressure, and the minimum pressure that
occurs during diastole is called the diastolic pressure.
The average arterial pressure during the cardiac cycle is the mean arterial pressure (MAP).
MEASURING ARTERIAL BLOOD PRESSURE 14 THE CARDIOVASCULAR SYSTEM: BLOOD VESSELS, BLOOD FLOW, AND BLOOD PRESSURE
o Pressure is usually measured in the brachial artery, which runs through the upper arm.
o Pressure measured in this manner is close to the aortic pressure because the brachial artery is not far
from the heart, and is also at about the same height as the aorta.
o Use a sphygmomanometer, which consists of an inflatable cuff and a pressure-measuring device that
displays air pressure inside the cuff, and they use a stethoscope placed over that brachial artery to listen
for sounds produced by turbulent blood flow.
o To measure blood pressure, the technician places the cuff around the upper arm and inflates it to
increase the cuff pressure.
o This pressure is transmitted through the tissue of the arm to the brachial artery, which runs to the
lower arm. .
o The technician increases cuff pressure until it is above systolic arterial pressure, which causes the artery
to collapse, stopping blood flow. Then allow the cuff pressure to fall slowly.
o When cuff pressure drops to where it is just slightly below systolic arterial pressure, the artery opens
briefly with each heartbeat when the pressure inside the artery is higher than that outside it, which
forces the vessel open.
o Turbulence creates sounds, called Korotkoff sounds, which can be heard through the stethoscope. When
the Korotkoff sounds first appear, the technician notes the cuff pressure and records it as systolic
o Eventually, cuff pressure falls just below diastolic arterial pressure, from which point the artery stays
open throughout the entire cardiac cycle because pressure inside the artery is always higher than that
o The technician notes the cuff pressure where sounds first disappear and records it as the diastolic
o Healthy individual is 110/70 (SP/SP)
The smallest arteries branch into even smaller arterioles, which lead either into a capillary bed or into
metarterioles, which then lead into capillary beds.
The walls of arterioles contain little elastic material but have an abundance of circular smooth muscle that forms
rings around the arterioles.
ARTERIOLES AND RESISTANCE TO BLOOD FLOW
The arterioles are the blood vessels that provide the greatest resistance to blood flow.
As blood flows from arteries to veins, pressure decreases, gradually.
A difference in pressure across any portion of the vasculature is called the pressure drop across that part.
The largest pressure drop occurs along the arterioles: 75-80 mmHg to 35-40 mmHg. It is related to high
resistance of arterioles.
The pressure becomes less pulsatile as it moves through the vasculature.
The major function of arterioles is to serve as points of control for regulating resistance to blood flow, which
serves 2 functions: 1) controlling blood flow to individual capillary beds, 2) regulating mean arterial pressure.
INTRINSIC CONTROL OF BLOOD FLOW DISTRIBUTION TO ORGANS
REGULATION IN RESPONSE TO CHANGES IN METABOLIC ACTIVITY: ACTIV HYPEREMIA
o Vascular smooth muscle cells in arterioles are sensitive to conditions in extracellular fluid and respond
to changes in the concentrations of a wide variety of chemical substances.
o Arteriolar smooth muscle either contracts or relaxes depending on whether concentrations of particular
substances rise or fall.
o Changes associated with increased metabolic activity generally cause vasodilation, whereas changes
associated with decreased metabolic activity induce vasoconstriction.
o When blood flow is matched to the tissue’s metabolic needs, the concentrations of oxygen and carbon
dioxide in the tissue are in a steady state: the rate at which oxygen enters the tissue from the blood
equals the rate at which it is consumed by the cells, and the rate of carbon dioxide entering the blood
equals the rate at which it is produced by the cells. 14 THE CARDIOVASCULAR SYSTEM: BLOOD VESSELS, BLOOD FLOW, AND BLOOD PRESSURE
o Metabolic rate increases, it causes a decrease in tissue oxygen (hypoxia) and an increase in tissue
o Initially blood blow is insufficient to keep up with metabolic demand, a condition called ischemia. The
decrease in oxygen and the increase in carbon dioxide both act on arteriolar smooth muscle, causing it
o Increase in blood flow following an increase in metabolic activity is termed active hyperemia (intrinsic
control). As a result of this increase in blood flow, the oxygen delivery to the tissue and carbon dioxide
removal from the tissue increase, and eventually a new steady state is achieved.
REGULATION IN RESPONSE TO CHANGES IN BLOOD FLOW: REACTIVE HYPEREMIA
o Tissue oxygen and metabolite concentrations can also change as a result of changes in blood flow.
o If blood flow is blocked or reduced below adequate levels for any reason, the oxygen concentration falls
and the carbon dioxide level rises because rates of oxygen consumption and carbon dioxide production
exceed rates of delivery and removal, respectively.
o Both induce vasodilation and a reduction in vascular resistance, which tend to increase blood flow.
o Once blockage is removed, the rate of flow will be higher than normal and will remain elevated until
excess metabolites are removed and tissue oxygen concentration is restored to normal (reactive
o If blood flow rises above what is required for metabolic needs, intrinsic control mechanisms will induce
vasoconstriction and a reduction in blood flow.
REGULATION IN RESPONSE TO STRETCH OF ARTERIOLAR SMOOTH MUSCLE: MYOGENIC RESPONSE
o Certain tissues contain arteriolar smooth muscle called stretchsensitive fibers (responsive to stretch,
when pressure of blood within the arterioles increases)
o A change in vascular resistance that occurs in response to stretch of blood vessels, and that does not
require the action of sympathetic nerves, bloodborne hormones, or other chemical agents, is a myogenic
o Perfusion pressure: the pressure gradient that drives blood flow through a given organ or tissue. It is
increases in an organ or tissue, blood flow increases and arteriolar pressure rises, which stretches the
o In arterioles containing stretchsensitive smooth muscle, the muscle fibers then contract, which
increases the arteriole’s resistance and decreases the flow of blood through it.
o A decrease in perfusion pressure brings about the opposite response – vasodilation and an increase in
o The variable that remains constant in myogenic control is blood flow. Local regulation that tends to
keep blood flow constant is called flow autoregulation.
o These changes in concentration cause vasoconstriction, which reduces blood flow. But the increase in
perfusion pressure also increases stretch of arteriolar walls, which in turn induces vasoconstriction and
a reduction in blood flow.
REGULATION BY LOCALLY SECRETED CHEMICAL MESSENGERS
o Blood vessel endothelial cells or cells in surrounding tissue secrete chemicals that effect contractile
activity of vascular smooth muscle.
o Nitric oxide, which is released on a continual basis by endothelial cells in arterioles and acts on smooth
muscle to promote vasodilation.
o Substances produced by inflamed tissues, such as bradykinin and histamine stimulate nitric oxide
o Prostacyclin, an eicosanoid that functions in preventing blood clots.
Independent Regulation of Blood Flow
Blood flow to organs is independently regulated by comparing the proportions of total blood flow each organ
receives at rest and during exercise.
If blood flow to organs were not regulated independently, the proportion of CO each organ receives would
remain constant, and blood flow to every organ would rise as cardiac output rises during exercise.
When the body makes a transition from rest to exercise, intrinsic controls divert blood away from the liver and
gastrointestinal tract and toward the muscles, which have high metabolic demands during exercise.
The increased blood supply to muscles provides the oxygen and nutrients needed to generate contractile force. 14 THE CARDIOVASCULAR SYSTEM: BLOOD VESSELS, BLOOD FLOW, AND BLOOD PRESSURE
Capillaries and Venules
Capillaries are the smallest blood vessels, measuring only 1mm long and 5-10 m, consisting of a single layer of
endothelial cells surrounded by a basement membrane.
The small diameter and thin wall provide a small diffusion distance between blood and the surrounding
Extensive branching results in 10-40 billion capillaries. A surface area of 600 sq. meters. Almost all cells of the
body are within 1mm of a capillary.
As blood enters capillary beds, the velocity of blood flow decreases.
Velocity of blood flow through the capillaries is about 0.1 mm/sec, allowing approximately 1 sec for exchange
between blood and tissue to take place.
Leakiness of the capillary wall enhances exchange between blood and tissue. Most capillaries, however, are
considerably “leaky” and are classified as either continuous or fenestrated, based on their degree of “leakiness”.
o Continuous capillaries have endothelial cells joined together such that the spaces between them are
relatively narrow. They are highly permeable to substances having small molecular sizes and/or high
lipid solubility and are somewhat less permeable to small water-soluble substances.
o The permeability of continuous capillaries to proteins and other macromolecules I very low because
these substances can neither readily cross membranes of endothelial cells, nor easily penetrate the gaps